Question CI Performance Databases 1000+ User

Yes, that will come down to how optimized your database is, whether it has appropriate indexes on columns, and the queries used. I have millions of records in about 30 tables for a large site and queries take a few milliseconds. An index for your primary ID in each table won't be nearly enough.

The performance problems won't really show up until you have a lot of records to sort through. Without indexes, mysql has to scan the entire table to find what you're searching for. Indexes fix that like a index in a book, by telling mysql which rows contain the relevant information so it knows where to look rather than having to inspect every row. The larger the db grows, the more important it is to have it properly indexed. You'd want to index all fields that you ORDER BY, GROUP BY, JOIN on.